The present invention relates to a stepping motor suitably applied to a driving motor for a timepiece using a quartz oscillator.
So-called quartz timepieces using a quartz oscillator have been popular. Some quartz timepieces, e.g., timepieces equipped at non-accessible locations, e.g., in station precincts, on wall surfaces, and the like are required to quickly and accurately correct time by a remote operation. As a motor applied to a quartz timepiece which can be remote-operated, a two-phase excitation type stepping motor which can be rotated in opposite directions by controlling phases of signals supplied to an excitation coil is known.
FIGS. 6 and 7 show a conventional two-phase excitation type stepping motor for a timepiece. FIG. 6 is a plan view of the stepping motor, and FIG. 7 is a plan view thereof.
In FIGS. 6 and 7, a rotor magnet 1 is formed of a columnar ferrite Magnetic poles N1, S1, N2, S2, N3, and S3 are magnetized at equal angular intervals (60.degree.) on the outer surface of the rotor magnet 1 so that different magnetic poles alternately appear. The rotor magnet 1 is rotatably supported by receiving plates 21a and 21b (FIG. 7) fixed to a rotor shaft 20.
Reference numerals 22 and 23 denote flat stators, formed of a planar material, for forming a closed circuit through the rotor magnet 1. Semi-circular recesses are formed in stator magnetic pole portions 22a and 23a of the stators 22 and 23. The semi-circular recesses oppose the outer surface of the rotor magnet 1 to have gaps therebetween.
Gap portions 24a and 24b whose two-end portions oppose each other to have a predetermined gap therebetween are formed on magnetic pole ends of the stator magnetic pole portions 22a and 23a at positions symmetrical about the rotational center of the rotor magnet 1. Two pairs of notch portions 22b and 23b, and 22c and 23c are formed in the inner surfaces of the recesses of the stator magnetic pole portions 22a and 23a at positions symmetrical about the rotational center. The notch portion 22b is formed at a 45.degree. position in a clockwise direction to have its opposing central line 100 as the center. The notch portion 22c is formed at a 15.degree. position in a counterclockwise direction to have the opposing central line 100 as the center. The gap portion 24a is formed at a 75.degree. position in the counterclockwise direction having the opposing central line 100 as the center. The still position of the rotor magnet 1 is determined by the positions of the notch portions 22b, 22c, 23b, and 23c. In FIG. 6, the rotor magnet 1 stands still between the magnetic pole portions of the stators while the notch portions 22b, 22c, 23b, and 23c are located respectively between the magnetic poles S3 and N1, between N1 and S1, between N2 and S2, and between S2 and N3.
A coil 25b wound around a bobbin 25a is provided to base portions 22d and 23d as ends opposite to the stator magnetic portions 22a and 23a of the stators 22 and 23. When he coil 25b is energized, the stator magnetic pole portions 22a and 23a are energized.
An upper stator assembly has been described. A lower stator assembly has the same structure as the upper stator assembly. Reference numerals 26 and 27 denote stators; 26b, 26c, 27b, and 27c, notch portions; 26a and 27a, stator magnetic pole portions; 26d and 27d, base portions; 28a and 28b, gap portions; 29a, a bobbin; and 29b, a coil. The gap portions 28b and 28a of the lower stator assembly are located at position symmetrical with the gap portions 24a and 24b of the upper stator assembly about the opposing central line 100. The notch portions 26b, 26c, 27b, and 27c of the lower stator assembly are located at positions symmetrical with the notch portions 22b, 22c, 23b, and 23c of the upper stator assembly about the opposing central line 100. More specifically, angles defined by the notch portions 22b and 26b and by the notch portions 23 b and 27b of the notch portions, four each formed in the upper and lower stator assemblies, for determining rotor still positions are set at 90.degree. to have the opposing central line 100 as the center, and angles defined by the notch portions 22c and 26c and by the notch portions 23c and 27c are set at 30.degree. to have the opposing central line 100 as the center.
Note that the stators 22 and 23 and the stators 26 and 27 are held by intermediate plates 20a of a nonmagnetic material to be separated at a predetermined distance in an axial direction. As a result, the stator magnetic pole portions 22a and 23a oppose the upper portion of the rotor magnet 1, and the stator magnetic pole portions 26a and 27a oppose the lower portion of the rotor magnet 1.
In this structure, when the coils 25b and 29b are energized to 2-2-phase excite the stators 22, 23, 26, and 27, four excitation patterns, i.e., (N, S, N, S), (N, S, S, N), (S, N, S, N), and (S, N, N, S) can be obtained.
In the third pattern, i.e., when the stators 22 and 26 are excited in the S pole and the stators 23 and 27 are excited in the N pole, the strength and direction of a magnetic field generated by the stators, and the strength and direction of a synthesized vector are as shown in FIG. 8. In FIG. 8, reference symbol .alpha. denote a magnetic field strength from the N pole to the S pole in the upper stator assembly (the stators 22 and 23); and .alpha.', a magnetic field strength from the N pole to the S pole in the lower stator assembly (the stators 26 and 27). The strengths of the two magnetic fields are equal to each other, i.e., .alpha.=.alpha.'. These magnetic fields are inclined at an angle .theta. in clockwise and counterclockwise directions with respect to the Y-axis (opposing central line 100). Reference numeral .beta. denotes a synthesized vector of the magnetic fields .alpha. and .alpha.'.
When the direction of a current to be supplied to the coil 29b is reversed so that the polarities of only the lower stators 17 and 18 are inverted, i.e., the stators 26 and 27 are excited to be magnetized in the N and S poles, respectively (the fourth excitation pattern). The strengths .alpha. and .alpha.' and directions of the magnetic fields and the strength and direction of a synthesized vector .beta.' are as shown in FIG. 9.
Therefore, the direction of the synthesized vector .beta. shown in FIG. 8 is changed clockwise through 90.degree., and the rotor magnet 1 is rotated clockwise through 30.degree.. When the fourth excitation pattern is switched to the first excitation pattern, the synthesized vector .beta. is further rotated clockwise through 90.degree. from the state shown in FIG. 9. When the first excitation pattern is switched to the second excitation pattern, the synthesized vector is further rotated clockwise through 90.degree.. In this manner, when the excitation patterns are successively changed, the rotor magnet 1 can be continuously rotated in a predetermined direction. Note that when the switching order of the excitation pattern is reversed to that described above, the rotor magnet 1 is rotated counterclockwise.
Every time the synthesized vector is rotated through 90.degree., the rotor magnet 1 is rotated stepwise through 30.degree.. The stop position after the 30.degree. rotation is determined by the positions of the notch portions.
In the conventional stepping motor for a timepiece, in consideration of the excitation patterns of the stators, the direction of the synthesized vector .beta. of the magnetic field strengths .alpha. and .alpha.' is changed through 90.degree., and its magnitude is also changed (.beta..apprxeq.3.7.beta.'). Therefore, a rotational torque applied to the rotor magnet 1 varies every time the rotor magnet is rotated through 90.degree., resulting in poor stepping angle precision as a timepiece. In particular, when the magnitude of the synthesized vector .beta. is changed, the rotor magnet which can be easily rotated by a large synthesized vector may not often be rotated by a small synthesized vector. For this reason, a torque for rotating the magnet by a small synthesized vector must be set. In this case, in order to obtain a desired torque, a rotor magnet of an expensive material must be used, or a stator material, windings or specifications of windings must be changed, resulting in an expensive timepiece.
Therefore, it is required to obtain a constant synthesized vector even when the excitation patterns are switched.
As a current to be supplied to the coil, rectangular pulses generated by a pulse generator are used. When a pulse current having a duty factor of 50% is used, a sufficient drive current is flowed. For this reason, after the rotor magnet is rotated through 30.degree., a magnetic force from the stators acts on the rotor magnet as a braking force, thus stopping the rotor magnet 1 at the correct position. However, when the stepping motor is used in a timepiece, a pulse current having a small pulse width (i.e., a small duty factor) is used since current consumption must be minimized. In this case, since the braking force is decreased, the rotor magnet cannot be stopped at a 30.degree. correct position by the conventional formed notch portions, and may be stopped at a 15.degree. or 45.degree. position. Since the stepping motor for a timepiece drives a second hand of a timepiece, if the stop position of the rotor magnet varies upon every stop, this variation directly influences movement of the second hand, resulting in poor quality as a timepiece.